1. Field of the Invention
The present invention relates to an exposure head and in particular to, an exposure head that exposes a photosensitive material with light beam which is modulated, according to image data, by a digital micromirror device (DMD).
2. Description of the Related Art
Conventionally, a DMD is a mirror device in which a large number of micromirrors with angles of their reflection surfaces being varied according to control signals are arranged two-dimensionally on a semiconductor substrate such as a silicon or the like. An exposure device using this DMD is structured, as shown in FIG. 15, by a light source 1 for irradiating laser light, a lens system 2 for collimating the laser light irradiated from the light source 1, a DMD 3 disposed at a substantial focus position of the lens system 2 and lens systems 4 and 6 for imaging the laser light reflected by the DMD 3 onto a scanning surface 5. According to such exposure device, each of the micromirrors for the DMD 3 is on-off controlled by an unillustrated control device depending on a control signal generated according to image data or the like in order to modulate laser light, whereby image exposure is performed by the modulated laser light.
The light source 1 is structured as follows. Namely, as shown in FIG. 16, a plurality of units each of which includes a single semiconductor laser 7, a single multi-mode optical fiber 8 and a pair of collimator lenses 9 for collimating laser light irradiated from the semiconductor laser 7 to bind on the end surface of the multi-mode optical fiber 8 are placed and a plurality of the multi-mode optical fibers 8 are bundled, so that a bundled fiber light source is formed.
A laser with around 30 mW of output is usually used for the semiconductor laser 7. An optical fiber with 50 μm of core diameter, 25 μm of clad diameter and 0.2 of NA (numerical aperture) is used for the multi-mode optical fiber 8. Thus, in order to obtain about 1 W of output, 48 (8×6) multi-mode optical fiber units 8 each of which has the above-described structure must be bundled, and the diameter of luminous point is about 1 mm.
According to conventional light sources, however, there arise problems that the diameter of luminous point becomes large when a plurality of optical fibers are bundled and thus sufficient focal depth cannot be obtained in a case in which exposure heads with high resolution are to be structured. Especially when only a part of DMD area is used, laser light must be condensed because the laser light is to be irradiated on a narrow area. As a result, sufficient focal depth cannot be obtained.
There is provided a method in which a focal depth is adjusted by performing autofocus by moving an imaging lens. Nevertheless, if an autofocus mechanism is provided, demerits such as an increase in costs and deterioration of vibration resistance characteristic may occur.
The invention was developed in order to solve the above-described problems and an object of the invention is to provide an exposure head that is capable of obtaining deep focal depth without providing an autofocus mechanism.